The present disclosure relates to subsea pump motor control, for example, control of electrical power flow on a permanent magnet motor in a subsea pump system located at a considerably long distance from the power source, and more particularly to a method and apparatus for starting up and controlling the torque speed characteristic of a such motor load system, specifically a subsea motor-pump system, driven by an on-shore drive in the absence of closed loop sensors at the subsea location.
Most subsea control of subsea pumps are based on induction motor driven systems. Some challenges raised by a permanent magnet motor, specifically a surface mounted magnet design, are related to the presence of an existing magnetic field in the rotor and the difficulty to synchronize, in the absence of a rotor position control sensor, the rotor position with respect to stator excitation defined by current and phase settings of a variable frequency or speed drive (VSD). One aspect that was also extensively debated in technical papers, is the possibility of a rotor back and forth oscillation during the startup and its impact on the power flow. While some of these theoretical aspects were debated in published technical papers, the methodology and algorithms of how to accomplish these were not yet known.
In some instances the startup currents for induction machines are, when there is present a breakout torque, in excess of the steady state operational requirements for those torque/speed motor pump characteristics. The possibility of saturating the transformers part of the transmission line leads to an over sizing of these components.
Like reference symbols in the various drawings indicate like elements.